The present invention relates to the field of power switches and more particularly of low loss power switches.
Often used power switches comprise bipolar transistors, MOS transistors and Insulated Gate Bipolar Transistors (IGBT).
It is generally admitted that it is the bipolar transistor operating in saturation condition that has the least power loss in the on-state. Indeed, a MOS transistor has, in the on state, a resistive characteristic between drain and source. Therefore, the power loss between drain and source is proportional to the square of the drain current and the loss becomes important for high currents. As for the saturated bipolar transistor, i.e. a transistor, the base current of which is greater than the collector current divided by the gain of the transistor, has a voltage drop (Vce) between collector and emitter which is practically constant and of a low value (in the order of 0.3 to 0.5 volt). Similarly, IGBTs have a Vce voltage drop which is practically constant but higher than that of a bipolar transistor (in the order of 1 volt).
However, the bipolar transistors which are capable of switching very high currents generally have a low gain and need a high base current to become saturated. Thus, the advantage of the low consumption between collector and emitter is counter-balanced by the drawback of substantial power loss in the base control circuit. In opposition, the control of IGBTs and MOS transistors is achieved upon a capacitive gate and the power needed for their control is negligible.
If a bipolar transistor is provided for switching a predetermined maximum current, its base control circuit will be designed for providing a current saturating the transistor when this maximum current flows through the latter. As previously mentioned, this base current is relatively important and it is generally still provided to the transistor when the current to switch becomes low. Thus, the power needed for controlling the transistor when it switches a lower current than intended is uselessly great and the efficiency of the transistor switch drops.